460 water neck

I just purchased a Ford Motorsport water neck (chrome with o-ring). It has a threaded hole in it that I believe is for a temp. controlled vacuum port (emissions).
My problem is that I need to plug this hole.
I tried to us a 3/8 npt plug but it is a little to big. I can't seem to find the right size plug.
Does anyone know the correct size of this hole or should I just tap it for the 3/8 plug.
Any help or suggestions would be great.

Thanks in advance

I would think it is 1/4 npt.
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I should have mentioned this initially
I also tried:
1/4" npt = to small
-6 sae = to small
-7 sae = to big
It doesn't seem to be a standard size.
That is why I am having trouble.

The threads are 5/8-18. I know because I just checked the same part I had received from Summit yesterday after reading your post. I am not sure how I'll plug it, probably 3/8 inverted flare cap with some sealer. 'm also not sure what sort of sender this is made for, certainly not my old 429 stuff.

I had the same dilemma.I used a 3/8" inverted flare fitting and put a little JB weld around the threads. it has worked perfect for 3 years so far.
Bob
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I used a flat head brass plug which I think I got at napa.
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Mine came with a chrome plated plug and seal ring/gasket. Yours should have come that way also.
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